In conventional offset lithography, a printing master is employed with an oleophilic image pattern on a hydrophilic and oleophobic background. An oil based printing ink as well as a water based fountain solution are fed from suitable roller applicators. The fountain solution serves to wet the background areas so that the ink is accepted only in the image areas. The quality of printing tends to a great extent upon maintaining a suitable balance between the amounts of ink and fountain solution fed as well as preventing emulsification between the ink and fountain solution and also maintaining the hydrophilic/hydrophobic character of the printing plate throughout its working life. Because it is difficult to maintain optimum printing conditions, methods have been sought to overcome these difficulties inherent in conventional lithography.
It has recently been discovered that the requirement for a fountain solution can be obviated by employing a planographic plate having a silicone elastomeric layer. Because the silicone is not wetted by the printing ink, no fountain solution is required. Since conventional offset lithographic masters can be rapidly and inexpensively imaged by electrostatographically depositing and fusing a particulate image pattern, with ink receptive particles commonly referred to as "toner", it has been attempted to make waterless lithographic masters by a similar process. It has been found however, that the very properties that make silicone elastomers reject printing ink also make them reject other materials such as melted toner. A method developed to more firmly adhere the toner to the silicone is to deposit the toner image on a layer of silicone gum and then cure the silicone to an ink releasing elastomeric condition. While this method can produce excellent printing masters it is not well suited for the preparation of nonimaged masters which can be stored for extended periods prior to imaging. This is because virtually everything will adhere to the silicone gum, including dust from the atmosphere, thereby creating contaminated areas and potential background areas of the master which can provide ink receptive sites and therefore yield prints with unacceptable background. Further, silicone gum masters have a tendency to adhere to photoreceptive surfaces making it difficult to effect a clean transfer of toner images. In addition, packaging of silicone gum masters is difficult since contact with other surfaces may damage the soft coating. Finally, chemically reactive sites on the silicone gums available for crosslinking may also be attacked by components of the atmosphere leading to premature curing and making the resultant rubber unsuitable for toner adhesion. It is to this problem of providing storable nonimaged waterless lithographic masters to which this invention is directed.